Speedometer Error and Tire Size

Date: 07/13/99 at 09:48:29
From: Michael Sugarman
Subject: Speedometer error vs tire size
I have a car that is used in competitive events like rallies where
average speed and exact speed are the most critical factors. As I
change tire size, speedometer error comes into play. I need to know my
exact speed at a given rpm based on tire size.
I know the complete dimensions of the tire, the drive ratio of the
car, and how far the tire goes in a straight line per tire revolution.
How do I figure my speed in miles per hour?
Any help would be appreciated.
Michael

Date: 07/13/99 at 10:54:19
From: Doctor Rob
Subject: Re: Speedometer error vs tire size
I assume that "the drive ratio of the car" is the number of axle
revolutions per crankshaft revolution, and "rpm" is crankshaft
revolutions per minute.
Multiply the rpm times the drive ratio of the car times the distance
the tire goes in a straight line per revolution (in feet) divided by
88. The factor of 88 is to convert from feet per minute to miles
per hour. (88 = 5280 feet per mile/60 minutes per hour).
Of course, this is assuming that you have perfect traction, the
distance the tire goes per revolution is constant with speed, and all
the turns average out, and a lot of other factors are negligible.
This will fail if you have an automatic transmission, since the
rotations of the crankshaft cannot be directly tied to axle turns,
but I suppose you knew that.
- Doctor Rob, The Math Forum
http://mathforum.org/dr.math/